CRA states that convention expenses “historically” have been viewed as capital expenditures

S. 20(10) very oddly provides that the cost of up to two qualifying conventions can be deducted in computing business income notwithstanding s. 18(1)(b). Given that the Anglo-Persian line of cases established that employees and agents are not a capital asset (as they are more-or-less free to walk out the door at any time), it is implausible that a convention expense could be found to be a capital expenditure (although the Shaver case and Griffith, [1956] C.T.C. 47 did just that.)

When asked about this, CRA sort of waffled:

Where a taxpayer has incurred expenses related to a convention in order to earn income from a business and such expenses are not capital expenditures, such expenses may be deductible in computing its business income without reference to subsection 20(10).

Historically, the CRA's position with respect to convention expenses is that such expenditures are generally capital expenditures to which paragraph 18(1)(b) applies.

Neal Armstrong. Summary of 6 October 2017 APFF Roundtable, Q.12 under s. 20(10).